The present invention relates generally to a method of preparing thin, continuous, isolated polymer membranes and their use in medical wound dressing and membranes for separations.
Thin polymer films are widely used in industry and medicine. For example, in separations, as anticorrosion coatings, as antistatic coatings, in microelectronic, for lubrication, for biocompatibilization, for dialysis, and as sterile barriers.
Typically, thin polymer films are prepared using charged polymers, or polyelectrolytes, which are alternately deposited on a substrate. Specifically, a buildup of multilayers is accomplished by dipping, i.e., cycling a substrate between two reservoirs containing aqueous solutions of polyelectrolytes of opposite charge, with a rinse step in pure water following each immersion. Each cycle adds a layer of polymer via electrostatic forces to the oppositely-charged surface and reverses the surface charge thereby priming the film for the addition of the next layer. Films prepared in this manner tend to be uniform, follow the contours and irregularities of the substrate and have thicknesses of about 10 to about 10,000 nm. The thickness of the films depends on many factors, including the number of layers deposited, the ionic strength of the solutions, the types of polymers, the deposition time and the solvent used. Although studies have shown that the substantial interpenetration of the individual polymer layers results in little composition variation over the thickness of the film, these polymer thin films are, nevertheless, termed polyelectrolyte multilayers (“PEMs”).
Though recently developed, PEMs are widely used in several fields, including light emitting devices, nonlinear optics, sensors, enzyme active thin films, electrochromics, conductive coatings, patterning, analytical separations, anticorrosion coatings, antistatic coatings, lubricating films, biocompatibilization, dialysis, and as selective membranes for the separation of gasses and dissolved species. PEMs are particularly suited for use as selective membranes because they are uniform, rugged, easily prepared on a variety of substrates, continuous, resistant to protein adsorption, have reproducible thicknesses, can be made very thin to allow high permeation rates and can be made from a wide range of compositions.
Although PEMs are widely used, certain limitations on their use have been imposed by the requirement that they be fabricated on a porous support. For example, the thickness of the films are somewhat inhomogeneous, with areas over the substrate's pores being thinner than the surrounding membrane. Furthermore, if separation from the support is desired, it is advantageous to separate a membrane from its support in an efficient manner. One method for separating a support and membrane is by dissolving the support in an organic solvent. For example, a cellulose acetate support may be dissolved with acetone to remove it from a multilayer comprising charged particles and polymers. See Mamedov and Kotov, Langmuir, 16, 5530 (2000). This process, however, is slow, requires disposal of organic solvents, destroys the substrate, may be difficult or impossible to employ on a multilayer membrane which does not contain charged particles and may denature, or deactivate, biologically-derived species (e.g., enzymes) incorporated within the membrane. Thus, a need continues to exist for a method of creating polyelectrolyte multilayer films which can be quickly and easily separated from their substrates without damaging the membranes or elements within the membranes, without destroying the substrates and without creating a waste stream of organic solvents.